A Marine Corps helicopter has made a “hard landing” in South Korea, military officials said.

The Marine CH-53E helicopter made the “hard landing” Tuesday during U.S.-South Korean military drills, according to a statement from United States Forces Korea.

Twenty-one personnel were on board the helicopter, including five crew members, the statement said. All were taken to a hospital, but 15 were quickly released. The remaining six were in stable condition.

The joint exercises are scheduled to end April 30.

The incident came as tensions continue to grow on the Korean peninsula.

North Korea has warned that the situation has grown so tense it cannot guarantee the safety of foreigners in the country and said embassies in Pyongyang should think about their evacuation plans.

But British Foreign Secretary William Hague said Monday that although there is reason for concern over the “frenetic and bellicose” rhetoric, Britain believes there has been “no immediate increased risk or danger” to those living in or travelling to North Korea.